E30 Zone

Does anyone know if I can use the M47 crank on original M42 rods and pistons? Or do I have to use shorter rods/forged rods/pistons?

Depends if you bore the block or not.
If you keep it 84mm you can use M42 pistons (not sure about rods) and you'll get a little over 1.9. BMW already make a 1.9 so it seems a little pointless using the M47 crank for that.

If you want to use the M47 crank to build a 2.0 or 2.1, you'll need a bigger bore (85, 86 or 87mm) and so different pistons.

I want to keep the bore. So it would be better to use an M44 crank with M42 pistons/rods? I am turbocharging the engine, just want a little bit more displacement.

For a 2l (1929cc), it says here that you can use the original pistons, but what about the rods?
http://www.e30zone.net/e30zonewiki/inde ... roker_Info

I'm not sure what rods you would need, I gave up looking into an M42 stroker as the cost vs gains didn't add up to me.

If you're turboing it, you will also need to consider what compression ratio you will end up with.
Tbh, I'd just boost the standard M42. The extra 100cc or so won't make a huge difference for the cost involved of getting it to a 1.9.

Then its probably better to drop it. The M44 has 1mm longer stroke, which I probably has to shave off of the piston. Do you know if the M42 has oil squirters?

M44 blocks have the piston crown squirters you could use an m44 block with the 83mm stroke and 85mm bore and use your head. This would give you 1900cc.
The m44 is also meant to have a better oil pump which may help with a turbo set up.

Is there a list of M44 parts that fit M42, and which parts on the M42 that are prone to failure?

Dude, forget the M44 if you want to go boost. The only M44 part you need on an M42 is the chain tensioner.

The M42 has a forged bottom end (M44 is cast) and the only thing stopping an M42 making decent turbo power is the 10:1 compression ratio - it apparently will still do upto 200bhp if mapped well.
Drop the compression to 8.5:1, add some ARP head bolts and it seems 250-300bhp is doable. Adding an extra 100cc or so will make little difference.

I don't need to use the oil pump from M44? The compression ratio is going to be dropped to about 7.5:1